Infinity sight view finder



May 10, 1949. P. MAINARDI ETAL INFINITY SIGHT VIEW-FINDERS 3 Sheets-Sheet 1 Filed Aug. 6, 1947 x PRIOR ART Mqhard/ Ma/nard/ I NVE N T0 RS ORNEYJ May 10, 1949.

P. MAINARDI ETAL' INFINITY SIGHT VIEW FINDERS 5 Sheets-Sheet 2 Filed Aug. 6, 19.47

a RART MAX gfilal'nardlj M r Ma/nar'd/ IN VEN TORS BY 2 3/ I Z AT TORNEYJ May .10, 1949. MMNARD] ETAL 2,469,927

INFINITY SIGHT VIEW FINDERS Filed Aug. 6, 1947 3 Sheets-Sheet s Pom 0e! M/hardl' Marcu NMa/nar INVENTO J ATTORNEYS Patented May 10, 1949 TNT OFFICE INFINITY SIGHT VIEW FINDER Pompey Mainardi and Marcus N. Mainardi, Hawthorne, N. J.

Application August 6, 1947, Serial No. 766,678

9 Claims. 1

This invention relates to View finders and particularly to infinity sight type of View finders.

It has previously been proposed to employ the cut-off which occurs when viewing through an optical element at about the critical angle of refraction, to define the edges of the field of view of a view finder. This has the advantage of all infinity sights in that movement of the eye relative to the viewing device causes the cut-off line to shift along so that the frame appears in the same position at infinity. Such known systems have the disadvantage, however, that the edge of the field thus defined is not straight but is rather, very definitely curved.

The object of the present invention is to provide a similar system in which the edges of the field are uite straight.

It is common practice to refer to a field frame when describing a view finder because the original view finders merely consisted of some mechanical frame. Such a description is not quite proper since actually the effect one is trying to obtain is that represented by a rectangular pyramid having its apex at the eye of the observer, which pyramid corresponds to the field of view from a camera lens. The present invention will be described with reference to such a geometrical pyramid, the proper field of view as found by the view finder being that seen within the pyramid from the apex thereof. The four sides of the pyramid correspond to the four sides of the rectangular field of view. More specifically the field of View at infinity is the area bounded by the intersections of the pyramid sides with the plane at infinity. When a finite aperture is used or when the observers eye is moved about, the pyramid shifts slightly but in its various positions the intersection at infinity is still .the same. The sides of the pyramid are effectively but not absolutely fiat, for various reasons including an offset where the sides pass through a layer of optical refractive material. The dimensions of the field of view are defined as the angular dimensions of the pyramid at its apex. The present invention is required only for view finders operating over the fields corresponding to ordinary cameras, i. e. over fields in which the angular dimensions of the geometrical pyramid are both greater than ten degrees since at very small angles the curvature of the frame lines as it occurs in the prior art was not objectionable.

According to the invention a view finder includes a layer or block of light refractive material with parallel entrance and exit surfaces. In a preferred form of the invention, the entrance and exit surfaces are orthogonal to the optic axis of the finder which obviously corresponds to the central axis of the pyramid described above. Thus the block is a sort of window which is used either by itself or in combination with other optical elements. The entrance and exit faces are always parallel to each other but need not be fiat or orthogonal to the optic axis; they may conveniently be pyramidal in form when the whole unit is made up of a number of prisms but this is less preferable. In order to define each edge of the field of View, a fiat layer is located in the block with a lower refractive index than that of the adjacent material of the block. This layer (one for each side of the field) is located obliquely in the block at the critical optical angle to a sheet of rays forming a side of the pyramid and directed toward the apex as said sheet of rays is refracted through the block. Thus, light is transmitted only if it is angularly within the pyramid. Since all the rays passing through the block emerge in the same direction at which they entered, all rays within the pyramid and reaching the apex are angularly within the pyramid. It should be clear that a sheet of rays forming a side of a pyramid before refraction through the block does not form the side of a pyramid after refraction. Furthermore, upon refraction, the sheet of rays no longer forms an absolute plane.

A finder constructed as indicated above provides an infinity sight view finder with straight boundaries. However, if the fiat layer is an air layer, the above conditions cannot be fulfilled and the eye does not see a straight line but rather sees a curved line boundary. This is not itself sur prising when one notes that a line drawn from an eye situated in the dense medium to a fiat surface at the critical angle and then moved about the surface to remain at the critical angle, generates a cone intercepting the surface in a circle.

We have discovered, however, that this cone of light within the block of refractive material need not define a curved edge of the field of view as seen by the observer. In fact we have found for all practical purposes the edge may be made to appear substantially straight all the way to infinity in both directions, 1. c. all the way to a angle of view on each side of the optical axis. Obviously this is far beyond the need in ordinary practical instruments, and it is quite satisfactory to provide a device in which the apparent line is straight out to 45 on each side of the optical axis, allowing a total angular field of view of 90.

For reasons explained below, this result can be obtained by proper selection of the index of refraction of the flat layer relative to that of the adjacent material of the block. Of course the critical angle depends on the difference of the indices of refraction at the layer and hence as different materials are used, the angle is changed between the layer and the entrance and exit surfaces of the block, since the boundary must be made to appear in the same direction.

We have found that when this angle between the surfaces of the block and the layer is Aand the index of refraction of the layer is' N1 and that of the block material is N, the cut-off line appears substantially straight from the center thereof out to a point a little beyond the angle :s when By way of a specific example, if the block is made of glass having an index of refraction of 1.55 and the layer instead of being air as in the prior art is made of material such as calcium fluoride havin an index of refraction of about tan A 1.38, one can apply this formula out to an angle of zr=45 on each side of the optic axis which gives A:45 21, and the edge will not deviate from straightness more than /2 over the whole 90 field.

The operation of the invention will be fully understood from the followin description when read in connection with the accompanying drawmgs.

Fig. 1 illustrates a simple form of the prior art and Fig. 2 the field of view obtained therewith. V

Fig. 3 shows a direct modification of the arrangement shown in Fig. 1 to incorporate the present invention, Fig. 4 being the field of view obtained therewith.

Figs, 5 and 6 are included to explain the operation of the invention.

Fig. 7 shows a family of curves corresponding to variations in index of refraction of the critical layer.

Figs. 8 to 12 inclusive illustrate various preferred embodiments of our invention.

Figs. 13 to 15 inclusive illustrate modifications of the invention to combine it with view finders.

of the inverted Galilean telescope type.

Fig. 16 is to illustrate the terms used in mathematical derivations pertinentto the invention.

Fig. 17 schematically shows the combination of a critical angle viewfinder and a range finder; Fig. 18 shows the range finder field.

In Fig. l a glass plate It) is provided with polished notches ll oriented at the critical angle to the principal ray in the refracted sides of the desired pyramidal field of view. Since the notches l 1 contain air, the sides of the pyramid defined thereby are not fiat and the edges of the field of view l3, as seen in Fig. 2, are curved so as to have barrel distortion.

In Fig. 3 the block of optional material I5 is provided with layers '16 containing a material whose index of refraction is considerably greater than that of air but is still somewhat less than that of the block 15 or at least less than the portions of the block 15, which are adjacent to the layers IS. The layers I6 are oriented at the critlayer, but should not be more than .25 less than that of the adjacent parts of the block. Obviously the index of refraction at the center of the block is immaterial to the present invention and the center of the block may be made of any material. This arrangement results in a field of view whose edges appear to be absolutely straight. In practice, even the most exacting inspection fails to detect the residual deviation from straightness even though such residual deviation is known to exist because of the trigonometry of the relationship. Vfhen referring to that part of the block adjacent to the edge defining layers, we mean all of the block which transmits critical rays to the layers. If the layer extends some distance through the block, the permissible movement of the eye position is correspondingly extended and in certain embodiments of the invention described below the whole block of glass must, therefore, be considered as adjacent to the edge defining layer or layers.

Figs. 5, 6 and 7 explain how a critical angle phenomenon which is fundamentally a cone can turn out to give a straight line rather than a circle as seen by the eye of the observer.

In Fig. 5 two glass prisms 20 and 2| are separated by a layer 22 whose index of refraction is loWcr than that of the glass. A horizontal line 23 is indicated across the middle of the hypotenuse face of the prism 2| for reference. Light from a point 24 on the line 23 passing to the mid-point 25 of the exit face of the prism 2| is at the critical angle 26 measured between the ray and the normal 2! to the layer 22. That is, any light arriving, through the prism 20, at the point 24 at greater than the critical angle is reflected back into the prism 20 but light coming to the point 24 at less than this angle 26 is transmitted through layer 22 to and through the prism 2|. The critical ray is that from the point 24 to the point 25 which then emerges from the prism 2 l to pass to the point 30. The rays from the point 25 to any point on the circular curve 31 are all at the same angle to the surface 22, i. e. are at the critical angle. Therefore, if one'were to place ones eye at the point 25, one would see light coming through the layer 22 from all points inside the arc of the circle 3| but would see only a black area to the left of this circle (assui'ning, of coins that the only light is that coming into the prism 23 from behind). However, after refraction at the point 25 (because of the angle A of the prism 2!) these rays from the circle 3| to the point 25 emerge not on the surface of a cone, but rather on a surface of irregular shape which approximates a flat plane if the angle A and the index of refraction of the layer 22 are properly chosen. Thus the points 32 are all substantially directly below the point 30.

While the line 3il33 is the one most easily drawn when analyzing the effect of different indices of refraction, this is, of course, not the line seen by the observers eye which, as shown in Fig. 6, is located at the point 36 itself. The ray 2425 3 0 reaches the point and the eye of the observer as it did in Fig. 5. However, any other critical ray reaching the eye comes from a different curve system. For example, the ray 35 comes from a point comes from a point 36 on a curve 37 whose principal point 38 is in the same horizontal plane as a point 39 from which light passes to the eye 30. Since the points 32 in Fig.

5 are substantially directly below the point 30, a momentsconsideration will show that the point 39 in Fig. 6 is directly above the point 25. Thus the eye at the point 30 sees a vertical straight line 40 corresponding to the critical rays passing through the layer 22 and the eye 30 receives light from the right of the line 40 but not from the left thereof. Furthermore, since the prisms 20 and :2! together form a flat block with parallel entrance and exit faces, and since the layer 22 also has parallel faces, this envelope of critical rays tracked back through the prism 20 will give a sheet of rays in which each ray is parallel to its corresponding segment on the eye side of the prism 2|. The observer in practice is not at all concerned with the shape of this envelope of rays either inside the prisms 2|) and 2| or outside the prism 29. Since all the critical rays outside the prism 20 are parallel to the plane defined by the line 4|] and the eye position 30, they must meet this plane at infinity. In other words, these rays intersect the infinity plane in the same line as does the plane formed by the line 40 and the eye position, which intersection is the left boundary of the field of View. Since the sheet of rays through line 40 and eye position 3B form one side of the View finder pyramid discussed in detail above, the sheet of refracted rays inside the prism is also considered to be the side of the pyramid in the block.

If the layer 22 is of air, the resulting curve 40 has a shape such as shown at 4| in Fig. '7. If the index of the layer 22 is increased and the angle A is simultaneously increased in order to maintain the cut-off line at the edge of the field of View, the cut-off curve 40 successively takes the shapes 42, 43, 44, 45 and 46 shown in Fig. '7.

If one wants to have the point 50 directly above the point 5| as it is on curve 44 but is not on curve 4|, one must select the arrangement which gives the curve 44. This would mean that the cut-off line is substantially fiat out to the point 50 which covers a field of about 60 on each side of the axis. Camera View finders do not normally require this wide coverage and curve 45 is perhaps even better for finders covering 45 on each side of the axis. It will be noted that the curve 45 is substantially flat all the way from the point 5| to the point 52 although mathematically it is the point 53 which is directly above the point 5|. Mathematically it can be shown that the angle represented by the point 53 on curve 45 or the point 50 on curve 44 is m when /NF-sin :v-N cos x JW(1cos x) where N is the index of refraction of the prisms 20 and 2|, N1 is the index of refraction of the layer 22, A is the angle of the prism 2| and .II is the angle of elevation of the point 53 above the point 5|. In order to cover an angular field of view having angular dimensions a by b, the value of the angle cc should preferably be but may have any value between and a. If a field of view is considerably different from a square so that a differs considerably from b, then it is preferable to have the value of a: between tan A for both dimensions of the field. Since the prism 6 angle will be different in the two dimensions when the field of view is not square, the value of a: will necessarily be different, especially if the layers corresponding to the sides and top and bottom of the field all have the same index of refraction. Both values of the angle of elevation of the point 53 should be between 5 and For all practical purposes these results are obtained when the value of the index of refraction N1 of the layer is between N.10 and N-.25 where N is the index of refraction of the block.

Although all of the theory necessary for an understanding of the invention has been set forth in detail, Fig. 16 is included since it may be of interest to those wishing to delve further into the mathematical derivations. The curve 56 is a general boundary curve and the point D, E is any point thereon measured from the axis of the finder which intersects the prism face normally at point 55. The crossing over point has coordinates x as above discussed. The following pertinent equations may be derived directly, the symbols being those used throughout this specification.

The general equation of the apparent boundary curve is:

cos Aw/N sin E sin A N N sin D= cos E For minimum D (the most extreme right hand point on the curve) cos A= and These equations permit direct calculation of residual deviations from straightness, but for all practical purposes, boundaries according to the present invention are perfectly straight.

In Fig. 8 the block 60 is made up of a number of glass prisms with the layer 6| corresponding to the right hand side of the field pyramid as seen by the eye 65; 62 corresponds to the left hand edge of the field, 63 corresponds to the top of the field, and 64 corresponds to the bottom of the field. In Fig. 9 the right, left, top and bottom edges are provided respectively by layers 1|, "[2, 13 and 14 in the compound block 10. In Fig. 10 the corresponding layers are BI, 82, 83 and 84. In Fig. 11 the corresponding layers are 9|, 92, 93 and 94. Obviously there is a multitude of ways in which to arrange the low index layers in the block of refractive material. One simple way is shown in Fig. 12 wherein the layers form their correct position, eifectively at infinity asthe eye-moves about, but-parts of the linesare out off and the other lines extend beyond the "fieldof view. This is-hot' true'with the corresponding arrangement shown in Fig. '9 where all iour layers extend -to al1 sides-of the block.

"In Fig. 9 as'sho'wn the entrance andexit faces are fiat and orthogonal 'tdthe axis indicated as alineof sight to the eye. 'If'the *p'yramldalpri'sm i is removed, the'eXit face'is'pyramidal and'no *lo'nger parallel-to the-entrance face I6. However,

if this entrance face 16 is made pyramidal and "parallel to'the eiiit race e.-g.'=by'plaoingaprism similar to'but larger than in front'of the-whole unit, the device is still useful as a view finder although the image is offset along the diagonals of the field.

lt'has-been suggestedthat the lack of straight- =ness of the apparent edge of the field might be improved-"by curving the entrance and exit surfaces of the-block. This does not seem to be the case, however. It has been shown that if the two faces of prism 2I of Fig. 5 are planes, the boundary'appearsat infinity both with respect to focus and with respect to the movement of the eye. parallel bundle of rays from a point at infinity cannot all strike the fiat layer 22 at the critical angle. Similarly, if the exit race is curved, the boundary no longer appears-at-infinity Sincea bundle of critical parallel rays from the fiat layer cannot emerge as a parallel bundle. This finder then ceases to be an'infinity sight. On the other hand, such curved surface I ID as shownin Fig. 15 may be useful to compensate for the effect of the dioptric power in a Galilean telescope made up of a negative lens III and a positive eyepiece I I2. A simpler arrangement for doing the same thing =-is 'sho'wn in'Fig; 13*whereinthe block I I3 according to the present invention is placed-immediately ahead of the lens III and may take any of the forms shown in'Fig. 8 to Fig. l2. The system shown in Fig. 13 is a highly preferred one since the lens system restricts the bundles of rays from one side of the field to the corresponding half of the negative lens and also-to the corresponding low refractive layer. This overcomes the cutting off of the boundaries which -as'mentioned above is an objectiori to 'the Fig." ll-arrangement. i l-similarly "solves the problem-- by curving the low refractive la-y'er I'I4 in'the block I*I.5,-

which is again --placed between the lenses, but '-froma manufact'urihg point of view-thisis not a desirable arrangementbe'cause of the diffic'ulty of producing the cylindrical surfaces to hold the layer I14.

In Fig. 17, a viewfinder-according to the invention is combined with a'split-fieldrange finder, The prisms 1-20 and 12 I with a lcw indexlayer "122 between themrepresent that part of the view finder whichfo'rms the upper edge of the field of" ciosea in housing to prevent light-being re- If the front surface is curved-then any flected from the layer I22, a portion of this region beyond the field of view,--is employedfor range finding, the range finderfield being introduced by prisms I23 and I24. The-appearance of the whole field of view is shown in Fig. 18. The view finder field is indicated at I5I the-auxiliary field of view is labeled I50, and there'is a dividing line I52 between the two fields, as-is common in the critical angle beam splitters. The rest oi-the region beyond the field of view indicated-at I53 appears perfectly black since -no other light is admitted to the sides of the system.

In order to obtain propercut-ofi' of the lower edge of the auxiliary field I50 to prevent overlapping at the line I52, it is desirable to use the critical angle principle once again so as to have the same cut-off for all eye positions. That is, the cut-ofi' line I52 is effectively placed at infinity in the same way as the edges of the field of view. To do this, prisms 1'25 and I26 with a low index layer I2! between are positioned in the range finder deflected beam with the layer I21 efiectively parallel to the layer which defines the lower edgeof the field of view I5I. The layers which define the sides and lower edge of this field I5I are not shown in Fig. 17, but are shown in Figs. 8 to 15. The right hand viewing point of the range finder system consists of a prism I23 which receives light from the target being ranged and directs it through the prism unit and IEt to the prism I24 and thence adjacent the edge of the view finder system. For ranging purposes, prism I29 is carried on a mount I30 which is pivoted and the adjustment thereof is indicated by a long pointer 'I3I and a scale 132. The main line of sight of the View finder is indicated at Me, whereas the critical ray'from the boundary line I52 is indicated at MI.

The range finder operates in the same manner as any split field range finder, the unique features thereof being the utilization of the low indeX layer I22 for combining the range finder with the view finder and the addition of the layer I22 in order to locate the lower edge of the field I50 at infinity to match the upper edge Having thus described various arrangements of our invention, we wish to point out that it is not limited to the above described embodiments, but is of the scope of the appended claims.

We claim:

1. A viewfinder forfinding a rectangular field defined by a geometrical pyramid having angular dimensions a by b each greater than 10 at the apex which corresponds to the eye position, comprising a block oflight refractive mat'erial'wlth parallel entrance and exit surf-aces transverseto the central axis of the pyramid; a flatlayer of non gaseous material with a refractive index between :10 and .25 less than that of the adjacent material of the block corresponding'to each side of the rectangular field located obliquely in the block at the critical angle of refraction to the side of the pyramid in the block to transmit only light angularly within the pyramid characterized by tan A equaling -and tan B equaling where A is the angle between the layer and said -surfaces and N1 the index of refraction of the %and a and y is between b and I) 2. A View finder according to claim 1 in which the entrance and exit surfaces are flat and orthogonal to the central axis of the pyramid.

3. A view finder for finding a rectangular field defined by a geometrical pyramid having angular dimensions a by I) each greater than 10 at the apex which corresponds to the eye position, comprising a block of light refractive material with Q parallel entrance and exit surfaces transverse to the central axis of the pyramid, a fiat layer of non gaseous material with a refractive index between .10 and .25 less than that of the adjacent material of the block corresponding to each side block at the critical angle of refraction to the side of the pyramid in the block to transmit only light angularly within the pyramid, characterized by tan A substantially equaling /N sin N cos /N -N lcos and tan B substantially equaling N sin N cos where A is the angle between the layer and said surfaces and N1 the index of refraction of the layer for each of the a angle sides of the field, B and N2 are the corresponding angle and index for the 1) angle sides of the field, N is the index of the block material adjacent to the layers.

4. A view finder according to claim 3 in which the entrance and exit surfaces are fiat and orthogonal to the central axis of the pyramid.

5. A view finder for finding a rectangular field defined by a geometrical pyramid having angular dimensions a by b each greater than 10 at the apex which corresponds to the eye position, comprising a block of light refractive material with parallel entrance and exit surfaces transverse to the central axis of the pyramid, a fiat layer of non gaseous material with a refractive index between .10 and .25 less than that of the adjacent material of the block corresponding to each side of the rectangular field located obliquely 10 in the block at the critical angle of refraction to the side of the pyramid in the block to transmit only light angularly within the pyramid characterized by tan A equaling x/ cos a; /NN (1cos x) and tan B equaling x/Nfi -sin y-N cos y /N -N (1cos y) where N1 is the index of refraction of the four layers, N is the index of the block material adjacent to the layers, A and B are the angles between said surfaces and the layers for the a and 2) angle sides of the field respectively and :c and 'y are both between a b 5 and 6. A view finder according to claim 5 in which the entrance and exit surfaces are fiat and orthogonal to the central axis of the pyramid.

'7. A view finder according to claim 1 in which reflectors are provided for receiving light from of the rectangular field located obliquely in the a viewing point spaced from that of the view finder and for directing it to the back of one of the flat layers to be totally internally reflected to the eye position adjacent to the side of said geometrical pyramid to form with the view finder field a range finder split field.

8. A view finder according to claim 3 in which reflectors are provided for receiving light from a viewing point spaced from that of the View finder and for directing it to the back of one of the flat layers to be totally internally reflected to the eye position adjacent to the side of said geometrical pyramid to form with the view finder field a range finder split field.

9. A view finder according to claim 5 in which reflectors are provided for receiving light from a viewing point spaced from that of the view finder and for directing it to the back of one of the fiat layers to be totally internally reflected to the eye position adjacent to the side of said geometrical pyramid to form with the view finder field a range finder split field.

POMPEY MAINARDI. MARCUS N. MAINARDI.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,264,504 Bertele Dec. 2, 1941 FOREIGN PATENTS Number Country Date 454,064 Great Britain Sept. 23, 1936 Certificate of Correction Patent No. 2,469,927. May 10, 1949. POMPEY MAINARDI ET AL. It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 3, line 64, for optional read optical; column 4, line 69, after the word point, first occurrence, strike out comes from a point;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Oifioe. Signed and sealed this 18th day of October, A. D. 1949.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

Certificate of Correction Patent No. 2,469,927. May 10, 1949. POMPEY MAINARDI ET AL. It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 3, line 64, for optional read optical; column 4, line 69, after the Word point, first occurrence, strike out comes from a point and that the said Letters Patent should be read With these corrections therein that the same may conform to the record of the case in the Patent Oflice. Signed and sealed this 18th day of October, A. D. 1949.

THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

